1. Field of the Invention
The invention relates in general to a frequency adjustment method, and more particularly to a frequency adjustment method for a reference oscillation signal.
2. Description of the Related Art
Wideband Code Division Multiple Access (WCDMA) is a digitized 3rd-generation mobile communication technique. In a WCDMA system, before a transmitter (a base station) transmits data, narrowband signals are first spread to broadband signals through spread spectrum coding and scrambled through scramble coding, and are then transmitted to a receiver (a cell phone). The data is in a unit of bits, and a spread sequence is in a unit of chips.
To allow the receiver to restore baseband signals transmitted by the transmitter, a cell phone first needs to synchronize with the base station, or else an output of noise waveforms is likely obtained in the baseband signals restored by the cell phone due to unsynchronized timings of the receiver and the transmitter. Hence, a WCDMA system achieves the synchronizations in a code-domain and a time-domain through an initial cell search procedure.
In an initial cell search procedure of a WCDMA system, at the time when a cell phone is activated under unknown situations, the cell phone needs to first search for base stations and locate a base station having the strongest signal strength. Thus, after synchronizing with the timing of the base station and identifying scramble coding utilized by the base station, the cell phone is able to correctly communicate with the base station and restore the original baseband signals. For identification purposes, fb represents a frequency of the baseband signals, and fc represents a frequency of carrier signal.
A certain error is usually present in an oscillator adopted in a cell phone or at a base station, such that a carrier frequency offset (CFO) exists between the frequencies of the two. The carrier frequency offset is also referred to as a frequency error ferror. Therefore, to allow the cell phone to normally receive signals, a frequency fref of a reference oscillation signal generated by a local oscillator of the cell phone ought to be adjusted, so that the frequency fref of the reference oscillation signal can better approximate the frequency fc of the carrier signal.
In an initial cell search procedure of a WCDMA system, once an absolute radio-frequency channel number (ARFCN) is selected, correction is performed on the frequency of the reference oscillation signal generated by the local oscillator. A WCDMA system has a tolerable error range of approximately 3-13 ppm. That is, when the frequency of the carrier signal is 2 GHz, the tolerable frequency error range between the frequency fref of the reference oscillation signal and the frequency fc of the carrier signal is 6 kHz to 26 kHz.
Thus, in the initial cell search procedure, the frequency synchronization between the reference oscillation signal of the cell phone and the carrier signal of the base station, and an appropriate correction on the received signals to generate correct down-converted signals, are rather crucial links allowing normal operations of a WCDMA system.
In the prior art, at the same time when performing the initial cell search procedure, a coarse automatic frequency control (AFC) is adopted to perform an initial frequency retrieval. The frequency fref of the reference oscillation signal generated by the local oscillator is corrected according to a result of the initial frequency retrieval, so as to adjust the frequency error ferror=fc−fref between the frequency fref of the reference oscillation signal and the frequency fc of the carrier signal to be within ±3 ppm.
According to a planning of a WCDMA system, a code frame has a length of 10 ms, and has 15 slots each containing 2560 subcodes. To facilitate the process of the initial cell search procedure for the cell phone, the WCDMA system provides a primary synchronization channel (PSCH) for assisting the cell phone to complete the slot synchronization. A length of the PSCH is only 1/10 of that of an original slot, and the slots at other positions do not include any messages or data. Therefore, only the first 256 subcodes of each slot contain a set of designed primary synchronization sequence.
Since all base stations utilize the same PSCH sequence, and the cell phone is also stored in advance with a PSCH sequence, whether the cell phone correctly receives the PSCH sequence is then a method that the receiver adopts for positioning a slot boundary. Further, the receiver performs a correlation calculation on the PSCH sequence and determines the frequency error ferror according to the correlation result.
As the frequency error ferror between the reference oscillation signal and the carrier signal gets larger, an output value of a PSCH correlator becomes smaller. Hence, an approach of utilizing a size of an output of a PSCH correlator for determining the frequency error ferror is often implemented in the initial frequency retrieval of the WCDMA system.
FIG. 1A shows schematic diagram of signal transmission and signal processing of a receiver in a WCDMA system. The left of the diagram shows a baseband signal of a base station and a carrier signal generated by a base station oscillator 106.
Through a mixer 102, a baseband signal is up-converted through the carrier signal to generate a transmission signal. The mixer 102 may be regarded multiplying two signals. After the frequency fb of the baseband signal passes through the mixer 102, a mixed signal having a frequency fb±fc is generated at an output terminal of the mixer 102. Through a filter (not shown), the mixed signal is transmitted as fb+fc or fb−fc. The transmission signal is transmitted from an antenna of the base station via a mobile communication network 10 and then received by an antenna of the cell phone.
When the transmission signal is received as a received signal by the cell phone, a mixer 101 of the cell phone down-coverts the received signal by use of a reference oscillation signal generated by a local oscillator 105.
Theoretically, the frequency fref of the reference oscillation signal equals the frequency fc of the carrier signal, and so the baseband signal can be restored in intact from the received signal. However, quite the contrary, the signal obtained after down-conversion by use of the reference oscillation signal is different from the baseband signal initially transmitted at the transmitter. Differences between the down-converted signal at the receiver and the baseband signal at the transmitter may be accounted by variations in the transmission process and the signal processing.
For example, in the transmission process, a baseband signal x(n) transmitted from the base station may be affected by noises or signal interference (N). Assuming the baseband signal is x(n), and a signal restored by the mixer 101 of the cell phone is y(n), the signal outputted by the mixer 101 is theoretically a combination of the baseband signal x(n) and the interference (N).
Apart from the interference in the transmission process, the frequency fref of the reference oscillation signal is not entirely the same as the frequency fc of the carrier signal generated by the oscillator of the base station, and so an error exists between the two. Assuming the frequency fref of the reference oscillation signal generated by the local oscillator is an initial oscillation frequency forig(fref=forig), and the frequency of the carrier signal is fc, a demodulated signal y(n) is affected by the frequency error ferror(ferror=fref−fc=forig−fc).
To determine the frequency error ferror between the reference oscillation signal and the carrier signal, approaches for correcting the frequency of the reference oscillation signal are categorized into coarse correction and fine correction.
The coarse correction on the frequency is to perform an initial correction on the frequency fref of the reference oscillation signal so that the frequency error ferror is reduced to within a frequency scan step Δf. After performing the coarse correction, a fine correction is performed on the frequency fref of the reference oscillation signal. The coarse tuning on the frequency shall be discussed below.
The reference oscillation signal generated by the local oscillator needs to further undergo the fine correction after the coarse correction. Therefore, an unacceptable frequency error may still be resulted to lead to an imprecise calibration if an unsatisfactory result is rendered by the preceding frequency coarse correction. A detection rate represents a rate whether the frequency error ferror can be adjusted and corrected to zero in the subsequent fine correction.
In a conventional frequency coarse correction, a frequency scan section is divided into a plurality of scan frequencies fi, and the scan frequencies fi are utilized in sequence for testing. The scan frequency fi represents an ith scan frequency in the frequency scan section.
A predetermined frequency scan step Δf exists between the scan frequencies fi of the frequency scan section, and a correlation result yi may be obtained according to each of the scan frequencies fi in the frequency scan section. By comparing the correlation results yi, a maximum value ymax of the correlation results yi can be obtained. Further, when the correlation results yi is a maximum value, the corresponding scan frequency fi enables the frequency error ferror to approximate a minimum value that the frequency coarse correction can achieve.
More specifically, after comparing the values of the correlation results yi, the maximum value ymax can obtained to accordingly obtain the corrected oscillation frequency (fref=fi).
Equations shall be given for deriving the above approach. A received signal r(t) is expressed by Equation (1).r(t)=α(t)s(t−tb)exp(j2πfct)+n(t)  Equation (1)
In Equation (1), s(t) represents a primary synchronization sequence, which is the first 256 chips of a slot; α(t) represents Rayleigh fading, and has a value assumed to be frequency-flat for simplification purposes; tb represents a timing offset between a system timing and an air slot boundary; fc represents a frequency of the carrier signal; and n(t) represents a sum of noise and other interferences.
Next, the received signal r(t) is down-converted through the mixer 101 according to the frequency fref of the reference oscillation signal. It should be noted that, the frequency fref of the reference oscillation signal changes according to different scan frequencies fi (fref=fi).
More specifically, the frequency fref of the reference oscillation signal is an initial oscillation frequency forig at the beginning, and the initial oscillation frequency forig does not equal the frequency fb of the carrier signal. In response to different scan frequencies fi, the frequency fref of the reference oscillation signal for down-converting the received signal through the mixer also changes, such that output results of the down-converted received signal from the mixer 101 also change as the scan frequency fi changes.
Since the scan frequency fi is a known value, the correlation results corresponding to different scan frequencies fi can also be obtained. In Equation (2), the correlation result yi represents a situation that an output of the PSCH correlator is a maximum in different slot boundary candidates tm.
                              y          i                =                              max                          t              m                                ⁢                                                                ∫                                  t                  i                                                                      t                    i                                    +                  T                                            ⁢                                                r                  ⁡                                      (                    t                    )                                                  ×                                                      s                    *                                    ⁡                                      (                                          t                      -                                              t                        m                                                              )                                                  ⁢                                  exp                  ⁡                                      (                                                                  -                        j                                            ⁢                                                                                          ⁢                      2                      ⁢                      π                      ⁢                                                                                          ⁢                                              f                        i                                            ⁢                      t                                        )                                                  ⁢                                  ⅆ                  t                                                                                                    Equation        ⁢                                  ⁢                  (          2          )                    
Wherein, ti is a starting time of the correlation of the scan frequency fi. As observed from ti, a signal for calculating the correlation may vary as the scan frequency fi changes. T represents an integration period, which is substantially equal to a WCDMA slot.
It is concluded from Equation (2) that, given the Rayleigh fading α(t) is a constant; the correlation result yi outputted by the PSCH correlator has a maximum value when the scan frequency fi is most approximate to the frequency fc of the carrier signal. Therefore, the PSCH correlation calculation is performed on all of the scan frequencies fi, and then it is determined which of the scan frequency is most approximate to the frequency of the carrier signal (fi≈fc) according to the correlation results. Hence, the frequency fref=fi of the reference oscillation signal can be obtained according to Equation (3).
                                          f            ^                    i                =                              max                          f              i                                ⁢                      {                          y              i                        }                                              Equation        ⁢                                  ⁢                  (          3          )                    
Once the scan frequency fi that is most approximate to the frequency fc of the carrier signal is obtained, the conventional solution then utilizes the scan frequency fi to correct the frequency fref of the reference oscillation signal. That is, the reference oscillation signal generated by the local oscillator 107 is adjusted from the initial oscillation frequency fref=forig to the scan frequency fref=fi corresponding to the maximum correlation result.
That is to say, in the prior art, details of estimating the frequency error ferror between the initial oscillation frequency forig and the carrier frequency fc according to the calculation results of the PSCH correlator are as below.
The frequency scan section is first divided into a plurality of scan frequencies fi. The different scan frequencies fi are respectively utilized as the frequency fi of the reference oscillation signal, and it is observed which of the scan frequencies fi corresponds to a maximum value of the correlation results yi generated by the PSCH correlator.
Therefore, in the prior art, for a specific scan frequency fi corresponding to the maximum value of the correlation results yi generated by the PSCH correlator, the specific scan frequency fi is determined as the frequency most approximate to the carrier frequency fc in the frequency scan section. The frequency fref of the reference oscillation signal generated by the local oscillator is then adjusted from the initial oscillation frequency forig to the scan frequency fi (i.e., the adjusted oscillation frequency).
For example, the smallest scan frequency and the largest scan frequency in the frequency scan section are defined as having a difference range of −15.4 ppm to +15.4 ppm with the frequency forig of the initial oscillation signal. A step size of the frequency scan step Δf in the range is 1.4 ppm. Thus, within the frequency scan section between −15.4 ppm to +15.4 ppm centered by the frequency fong of the initial oscillation signal, the frequency scan section includes a total of (15.4*2)/1.4+1=23 scan frequencies when 1.4 ppm is a range of one frequency scan step Δf.
FIG. 1B shows a schematic diagram of a sequential frequency scanning approach in the prior art. For simplified illustrations, not all the 23 scan frequencies in the frequency scan section are depicted. Instead, the different scan frequencies are indicated in different numbers, and the calculation results obtained by the PSCH correlator from the scan frequencies are indicated in corresponding numbers. For example, a first correlation result y1 is obtained according to a first scan frequency f1, a second correlation result y2 is obtained according to a second scan frequency f2, and so forth.
In simple words, in a convention solution, scanning is performed by different scan frequencies fi within a frequency scan section. The approach of testing and verifying the change in scanning the scan frequencies fi one after another is utilized for frequency coarse correction of the reference oscillation signal.
However, the detection rate of the coarse correction based on sequentially calculating and comparing the correlation results of the scan frequencies may be unsatisfactory, such that the frequency error ferror may be greater than the frequency scan step Δf. That is to say, the conventional solution of coarse correction does not guarantee that the frequency error ferror can be reduced to 0 in the subsequent fine correction.
When adopting the above conventional solution that estimates the frequency most approximate to the frequency of the carrier signal for adjusting the oscillation frequency, a reason for not being able to correctly determine the frequency error in the fine correction is that, it is possible that many different scan frequencies fi correspond to larger correlation results yi, or a maximum absolute value at the same time corresponds to two scan frequencies that both have the maximum correlation results although being two extreme values with a great difference in between (e.g., f1 and f23). Further, as Rayleigh fading αt changes along with time, an outcome of the frequency error ferror estimated by Equation (3) is not at all ideal.
It is even discovered through simulation results that, the frequency correction results may diverge when performing the coarse correction on the reference oscillation signal according to the above conventional solution, such that an appropriate value for correcting the oscillation frequency is unlikely to be determined. Therefore, the conventional solution of scanning all the scan frequencies fi within the frequency scan section and directly determining and adjusting the reference oscillation signal according to the correlation results faces certain drawbacks.